Recent evolutions of wireless communications systems have resulted in a broad spectrum of standards, each standard specifying different frequencies and bandwidths for wireless communication. These multiple standards are expected to co-exist, and dynamic management of multiple wireless network standards has become essential. This calls for a shift from dedicated single carrier frequency radio systems to versatile and adaptive systems capable of efficiently handling wide spread frequency bands. In other words, future radio systems should be able to cope with dissimilar center frequencies and signal bandwidths imposed by diverse standards while maintaining competitive performance. In particular, power amplifiers of radio transceivers should be able to amplify signals for different standards and frequencies.
Referring to the drawing figures, there is shown in FIG. 1 a block diagram of a known Doherty amplifier 10 used to amplify signals in a wireless communication system. For example, the Doherty amplifier 10 of FIG. 1 may amplify signals to be transmitted by an antenna over an air interface. Also, the Doherty amplifier 10 of FIG. 1 may amplify signals received by an antenna from the air interface. The Doherty technique was first introduced in 1936. It is an approach for actively modulating the load seen by a main transistor of a power amplifier to improve its efficiency in the back-off condition via a judiciously configured auxiliary transistor.
The Doherty amplifier 10 has a main amplifier 12, an auxiliary amplifier 14, and an input power divider 16. The outputs of the main amplifier 12 and the auxiliary amplifier 14 are coupled to a combining network consisting of impedance inverters with characteristic impedance RT 30 and ZT 32. The main power amplifier 12 includes an input matching network 18, a power transistor 20 and an output matching network 22. Similarly, the auxiliary amplifier 14 includes an input matching network 24, a power transistor 26 and an output matching network 28.
The main amplifier 12 is Class AB biased and matched to ensure peak efficiency at a predetermined p-dB back-off that corresponds to a peak to average power ratio, PAPR, of the input signal. The input signal voltage at the specific back-off is given by
            V      in        =                  V                  in          ,          max                    p        ,where Vin,max is the maximum input voltage of the input signal. The auxiliary amplifier 14 is Class C biased and starts conducting at
      V    in    =                    V                  in          ,          max                    p        .  
In the high power region,
            I      aux              I      main        =            p      ⁢                          ⁢              α        ⁡                  (                      p            -            1                    )                            1      +                        (                      p            -            1                    )                ⁢        α            and the ratio of the two transistor currents is given by
            V      in        ≥                  V                  in          ,          max                    p        ,where α=0 at
      V    in    =            V              in        ,        max              p  and increases slowly to 1 when Vin=Vin,max. Given the input signal PABR, p, the combining network element parameters are given by:
            R      T        =          R      opt        ,          ⁢            Z              T        ⁢                                  ⁢        1              =                            R          opt                          p                    =                        R          T                          p                    Assuming that the transistors 20 and 26 are ideal, the impedances of the main amplifier 12 and the auxiliary amplifier 14 vary as a function of the input signal according to the following expressions:
      Z    main    =      {                                                                      pR                T                                                                    0                ≤                                  V                  in                                ≤                                                      V                                          in                      ,                      max                                                        p                                                                                                                          pR                  T                                ⁡                                  (                                      1                    -                                                                  α                        ⁡                                                  (                                                      p                            -                            1                                                    )                                                                                            1                        +                                                                              (                                                          p                              -                              1                                                        )                                                    ⁢                          α                                                                                                      )                                                                                                                          V                                          in                      ,                      max                                                        p                                ≤                                  V                  in                                ≤                                  V                                      in                    ,                    max                                                                                      ⁢                                  ⁢                  Z          aux                    =              {                                            ∞                                                      0                ≤                                  V                  in                                ≤                                                      V                                          in                      ,                      max                                                        p                                                                                                                          R                  T                                                  p                  ⁡                                      (                                          1                      +                                                                        1                          +                                                                                    (                                                              p                                -                                1                                                            )                                                        ⁢                            α                                                                                                                                p                            ⁡                                                          (                                                              p                                -                                1                                                            )                                                                                ⁢                          α                                                                                      )                                                                                                                                            V                                          in                      ,                      max                                                        p                                ≤                                  V                  in                                ≤                                  V                                      in                    ,                    max                                                                                          
To maintain good average efficiency, the load modulation of the Doherty amplifier 10 should be adjusted as a function of the PAPR. In addition, the auxiliary transistor 26 should supply a drain current (p−1) times larger than the current of the main transistor 20 at peak power. The resulting efficiency versus input power level varies as a function of frequency. As the frequency varies, the load modulation in the Doherty amplifier 10 is affected as a direct result of the variation of the electrical lengths of the impedance inverters with frequency. Hence, the impedance seen by the main transistor 20 deviates from the ideal case and consequently, the efficiency of the Doherty amplifier 10 deteriorates. Thus, the improper load modulation resulting from the frequency variation yields an accentuated degradation of efficiency versus frequency as the value of p increases.
Therefore, the conventional Doherty amplifier 10 does not perform well for multiple frequencies and multiple standards.